This invention relates to a filtration device and associated method for separating a slurry into solid and liquid phases. More particularly, this invention relates to a pressure filtration device and associated method for continuously separating solid phase material from a slurry and rapidly depressurizing that material.
In the production and processing of chemicals, it is frequently necessary to separate solid material from a slurry. For example, slurries are produced by chemical reactors and precipitation units. Solid material is typically separated from the slurry by filtration or centrifugal separation. Suitable filtration methods include gravitational, vacuum-assisted and pressure filtration. For example, in the production of purified terephthalic acid, crystals of terephthalic acid are conventionally separated from a slurry using a rotary vacuum filtration unit. The filtration unit has a cylindrical filter drum rotatably mounted inside a housing. As the slurry is fed into the filtration unit housing, a pressure differential is created inside the filter to move the liquid portion of the slurry through the filter media. The crystals are deposited in a layer on the outer cylindrical surface of the drum.
Following separation of the solid material from the slurry, the solid material removed from the filter media, typically as a cake of solid phase material, prior to subsequent processing or disposal. In some applications, a jet of pressurized gas removes the solid material from the filter media and pushes that material to a discharge outlet. Where pressure filtration is used, certain processes may require that the solid material is discharged to lower pressure (such as atmospheric conditions) for downstream processing. For continuous processing operations, it may also be desired that the pressure differential in the filtration unit not be significantly disrupted.
U.S. Pat. No. 5,093,001 discloses a pressurized filtration unit that has a pair of valves at the output to provide for the discharge of material. This patent is incorporated by reference herein. That configuration, however, is not sufficient to solve the pressure control problem and to maintain a high level of throughput of material through the filtration unit. U.S. Pat. Nos. 5,589,079 and 5,470,473 disdose a pair of valve units and hoppers that provide for batch processing of solid material from a pressurized filtration unit. These patents are incorporated by reference herein. Each valve unit may have a dome-shaped valve member. In operation, solid material is discharged from the filtration unit into a first hopper. A valve unit attached to the first hopper seals that hopper while the pressure is equalized between the first hopper and the second hopper. That pressure is intermediate the initial pressure and the final, lower pressure. After the pressure is equalized between the hoppers, the solid material is then transferred to the second hopper. The second hopper is depressurized to the final, lower pressure, and then the solid material is discharged to a storage vessel or conveyer. While the disclosed apparatus and method are superior to prior devices and methods, that apparatus and method are limited to batch processing of solid material. The multi-stage, batchwise processing requirements create a bottleneck that preclude high speed, continuous throughput of material. The valve units and pressure equalization equipment also require frequent maintenance to keep that equipment working efficiently. Thus, there is a need for a pressure filtration unit that provides for continuous processing of solid material. In particular, there is a need for a pressure filtration apparatus that provides for the continuous processing of slurry by rapidly decompressing solid phase material separated from the slurry while maintaining the pressure difference between the filtration unit and the downstream equipment.
An assembly for continuously recovering solid phase material from a slurry comprises, in accordance with the present invention, a pressurized filtration unit including a vessel having filter media for separating the solid phase material from the slurry. The filtration unit has a slurry input and, optionally, a source of inert gas. A liquid outlet discharges liquid phase material from that unit. Gas can also be discharged through the liquid outlet, depending on the design of the filtration unit. A solids outlet discharges solid phase material from the filtration unit. The filtration unit can optionally include a washing apparatus for dispensing a washing fluid or liquor onto a cake of material that is deposited on the filter media. The washing fluid is displaces other liquids and/or impurities in the cake.
A material transport continuously moves solid phase material from the solids outlet of the filtration unit to a depressurizing chamber. The depressurizing chamber has a lower pressure than the filtration unit to facilitate subsequent processing of that material. As the solid phase material is transported into the depressurizing chamber, that material is directly and rapidly depressurized to the lower pressure of that chamber. Advantageously, the rapid depressurization can cause xe2x80x9cflashingxe2x80x9d of the solid phase material, which removes residual liquid, gas and volatile substances from that material. The depressurizing chamber may further include one or more outlets for releasing the vapor, gas and/or volatile substances from that chamber. The depressurizing chamber can contain an inert gas to stabilize reactive or otherwise unstable solid phase material. Any suitable conveyance can remove the solid phase material from the chamber for subsequent processing, such as drying.
The material transport generally includes an inlet for receiving solid phase material from the solids outlet of the filtration unit. That material is discharged under pressure into the material transport inlet. It is a feature of the present invention that the material transport maintains the pressure difference between the filtration unit and the depressurizing vessel. The solid phase material is transported to an outlet where it is discharged into the depressurizing chamber. That material is rapidly depressurized as it is exposed to the lower pressure of the depressurizing chamber.
In another aspect, a system of one or more filtration units, depressurizing units and material transports is provided. Such a system can further include one or more chemical reactors or precipitators that provide a source of slurry to the filtration units. One or more downstream conveyors and/or dryers can aid in further processing of the solid phase material. The conveyor may be a screw conveyor or other type of conveyance.
In accordance with another aspect, a method for separating solid phase material from a slurry includes the steps of (a) providing a filtration unit having a vessel maintained at a first pressure and having filter media, (b) introducing a slurry to the vessel, (c) separating the slurry into a solid phase material and a liquid phase material, (d) continuously transporting the solid phase material to a depressurizing chamber while maintaining the pressure difference between the filtration unit and the chamber, and (e) rapidly depressurizing the solid phase material as it enters the depressurizing chamber. The solid phase material is maintained at substantially the first pressure until it enters the depressurizing chamber, although a minor amount of controlled pressure leakage during transportation of the solid phase material may be permitted. Further steps include (f) providing the slurry from a chemical reactor; and (g) conveying the depressurized solid phase material to a dryer.